title: Inhibition of JAK3 and STAT6 tyrosine phosphorylation by the immunosuppressive drug leflunomide leads to a block in IgG1 production.
abstract: Leflunomide is an immunosuppressive drug capable of inhibiting T and B cell responses in vivo. A number of studies demonstrate that leflunomide functions both as a pyrimidine synthesis inhibitor and as a tyrosine kinase inhibitor. We previously reported that leflunomide inhibits LPS-stimulated B cell proliferation, cell cycle progression, and IgM secretion. This inhibition can be reversed by the addition of exogenous uridine, suggesting that leflunomide functions as a pyrimidine synthesis inhibitor in B cells. We report here that while the addition of uridine restored proliferation and IgM secretion to leflunomide-treated LPS-stimulated B cells, as determined by metabolic labeling and immunoprecipitation, it did not completely restore secretion of IgG Ab. We hypothesized that leflunomide inhibits LPS-induced IgG secretion by inhibiting tyrosine kinase activity required for isotype switch. We tested this hypothesis in a well-defined model of isotype switch, LPS plus IL-4 induction of IgG1. Leflunomide inhibited IgG1 secretion in this model in a dose-dependent manner. The signal transduction pathway utilized by IL-4 to induce IgG1 involves tyrosine phosphorylation of the IL-4 receptor, JAK1, JAK3, and STAT6 proteins induced by IL-4 binding to the IL-4R. Leflunomide diminished the tyrosine phosphorylation of JAK3 and STAT6 in the absence or presence of uridine. In gel mobility shift studies, STAT6 binding to the STAT6 DNA binding site in the IgG1 promoter decreased in the presence of leflunomide or leflunomide plus uridine. Taken together, these data suggest that leflunomide acts as a tyrosine kinase inhibitor to block IgG1 production.
authors.affiliation: Department of Immunology/Microbiology, Rush-Presbyterian-St. Luke's Medical Center, Chicago, IL 60612, USA.
authors.forename: K
authors.initials: K
authors.lastname: Siemasko
authors.forename: A S
authors.initials: AS
authors.lastname: Chong
authors.forename: H M
authors.initials: HM
authors.lastname: Jäck
authors.forename: H
authors.initials: H
authors.lastname: Gong
authors.forename: J W
authors.initials: JW
authors.lastname: Williams
authors.forename: A
authors.initials: A
authors.lastname: Finnegan
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology / Q000201:enzymology* / Q000276:immunology / Q000378:metabolism; D001667:Binding, Competitive / Q000187:drug effects / Q000276:immunology; D004268:DNA-Binding Proteins / Q000037:antagonists & inhibitors / Q000276:immunology / Q000378:metabolism; D006131:Growth Inhibitors / Q000494:pharmacology; D007074:Immunoglobulin G / Q000096:biosynthesis* / Q000187:drug effects / Q000378:metabolism; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D015847:Interleukin-4 / Q000494:pharmacology; D007555:Isoxazoles / Q000494:pharmacology*; D053616:Janus Kinase 3; D000077339:Leflunomide; D008070:Lipopolysaccharides / Q000494:pharmacology; D008213:Lymphocyte Activation / Q000187:drug effects; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D010766:Phosphorylation / Q000187:drug effects; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D050800:STAT6 Transcription Factor; D015398:Signal Transduction / Q000187:drug effects / Q000276:immunology; D015534:Trans-Activators / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism*; D014529:Uridine / Q000494:pharmacology
chemical_list: D004268:DNA-Binding Proteins; D006131:Growth Inhibitors; D007074:Immunoglobulin G; D007166:Immunosuppressive Agents; D007555:Isoxazoles; D008070:Lipopolysaccharides; D050800:STAT6 Transcription Factor; C494100:Stat6 protein, mouse; D015534:Trans-Activators; D015847:Interleukin-4; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; C507935:Jak3 protein, mouse; D053616:Janus Kinase 3; D000077339:Leflunomide; D014529:Uridine